Room 355 EF

16:00 - 18:00

Moderators:

Vasily L. Yarnykh, Xiaohong Joe Zhou

Kay Nehrke¹, Maarten J. Versluis², Andrew Webb², and Peter Börnert^1,2
1Philips Research Laboratory, Hamburg, Germany, ²Leiden University Medical Center, Leiden, Netherlands

A new acquisition scheme based on virtual stimulated echoes has been developed for the DREAM B₁⁺ mapping approach, which allows compensation of both, T₂and T₂*, effects. The new option has been studied on phantoms and in vivo at 7T. For the phantom experiments, rubber phantoms were employed, showing that the sequence allows accurate B₁⁺ mapping for a short T₂ in the order of a millisecond. For the in vivo experiments, B₁⁺ maps of the brain were acquired, showing that the contrast between brain tissue and CSF is reduced for the new sequence option.

Naoharu Kobayashi¹ and Michael Garwood^2
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States, ²University of Minnesota, Minneapolis, Minnesota, United States

Obtaining the B1 map is necessary for a number of MRI methods, including T1 measurement. We introduce a new approach to actual flip angle imaging (AFI) that achieves 3D B1 mapping even when T2* is short. The approach utilizes COncurrent Dephasing and Excitation (CODE) with Gradient Offset Independent Adiabaticity (GOIA). CODE is a short TE radial GRE sequence that is relatively robust compared to some other ultra-short TE sequences. The use of GOIA pulses in CODE significantly reduces the peak power and SAR.

Marcin Jankiewicz¹ and William A. Grissom^2
1MRC/UCT Medical Imaging Research Unit, Department of Human Biology, University of Cape Town, Observatory, Western Cape, South Africa, ²Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, United States

We introduce a new hybrid excitation and off-resonance B₁⁺-encoding pulse that enables Bloch-Siegert B₁⁺ mapping with a much shorter TE, enabling mapping of short-T₂ species.

Jaco J.M. Zwanenburg¹, Fredy Visser^1,2, Jeroen Hendrikse¹, and Peter R. Luijten^1
1Radiology, University Medical Center Utrecht, Utrecht, Netherlands, ²Philips Healthcare, Best, Netherlands

In heavily T2-weighted FLAIR images at 7T, a lateral asymmetry was observed in the contrast of the pulvinar, a nucleus of the thalamus. Due to the small distance between the bilateral pulvinars, B1 inhomogeneity seemed unlikely as explanation for the asymmetry in contrast. In this work, however, we show with simulations and experiments that TSE trains exhibit a highly T2 dependent sensitivity to B1 inhomogeneity, which leads to asymmetry in image contrast (rather than in signal intensities alone). This finding urges for careful interpretation of TSE images (also at clinical field strengths), and highlights the need for precise B1 shimming.

Anuj Sharma¹, Samantha J. Holdsworth², Rafael O'Halloran², Eric Aboussouan², Anh Tu Van², Murat Aksoy², Julian R. Maclaren², Roland Bammer², Victor Andrew Stenger³, and William A. Grissom^1
1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, ²Radiology, Stanford University, Stanford, California, United States, ³Medicine, University of Hawaii, Honolulu, Hawaii, United States

An approach to designing patient-tailored 3D spokes excitation RF pulses that simultaneously excite multiple slices and compensate transmit RF field inhomogeneity at high static field is presented. An interleaved greedy and local optimization algorithm for spokes pulse design was extended to the multiband spokes pulse design problem. In-vivo results demonstrate the effectiveness of the designed multiband spokes pulses in reducing flip angle inhomogeneities caused by field inhomogeneity at 7T.

Kalina V. Jordanova¹, Dwight G. Nishimura¹, and Adam B. Kerr^1
1Electrical Engineering, Stanford University, Stanford, CA, United States

A novel phase-based B₁ estimation method using adiabatic refocusing (BEAR) is proposed and validated experimentally at 1.5T. The method exhibits a linear relationship between phase and B₁ that is insensitive to off-resonance, T₁, and T₂. Using this method, B₁ mapping can be localized to a slice or 3D volume, with a spin-echo acquisition that is appropriate for fast projection measurements. These properties make BEAR an ideal candidate for use in robust transmitter gain calibration of nominally homogeneous volumetric RF coils.

Ralf Kartäusch¹, Florian Fidler¹, Toni Drießle¹, Thomas Kampf², Thomas C. Basse-Lüsebrink^1,2, Uvo C. Hoelscher¹, Peter M. Jakob^1,2, and Xavier Helluy^2
1MRB Research Center for Magnetic-Resonance-Bavaria, Würzburg, Bavaria, Germany, ²EP5, Universität Würzburg, Würzburg, Bavaria, Germany

In this work we introduce a robust RF only spatial encoding method, which is mimicking conventional phase encoding using B0-gradients by exploiting the properties of spatially dependent Bloch-Siegert phase shifts induced by a RF gradient coil. Being a RF only encoding technique it has immunity against eddy currents and very short switching time. The application of BS-gradients are not restricted in principle to spatial phase encoding and could be for example applied to flow or diffusion measurements.

Qi Duan¹, Peter van Gelderen¹, and Jeff H. Duyn^1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, Maryland, United States

This abstract investigates fast simultaneous B₀/B₁⁺ mapping by Bloch-Siegert shift via lowering the off-resonance frequency of this pulse, as theoretical analysis indicated that the sensitivity of Bloch-Siegert based B₁⁺ mapping can be substantially improved when irradiating closer to resonance. Using optimized irradiation pulse shape and gradient crushers to minimize direct excitation effects, in vivo experiments on human brain at 7T confirmed the improved sensitivity available with this approach operating with peak B₁⁺ much larger than the frequency offset. This improved sensitivity translated into an 80% reduction in B₁⁺ estimation errors, without increasing tissue heating.

Tiejun Zhao¹, Yujuan Zhao², Anthony DeFranco², and Tamer S. Ibrahim^2
1Siemens Healthcare; Siemens Medical Solutions USA, Inc., Pittsburgh, PA, United States, ²University of Pittsburgh, Pittsburgh, PA, United States

Multi-band (MB) excitation for data acquisition was successfully incorporated into the Bloch-Siegert Shift B1 mapping sequence. The B1 maps from the MB excitation were validated against the conventional Bloch-Siegert data acquisition and were found to be highly comparable with conventional Bloch-Siegert method. The SAR reduction with a factor of two and the slice coverage increased by three folds were demonstrated with the MB methods.

Bogdan G. Mitrea¹, Axel J. Krafft¹, Ruitian Song¹, Ralf B. Loeffler¹, and Claudia M. Hillenbrand^1
1Radiological Sciences, St Jude Children's Research Hospital, Memphis, TN, United States

Spin-lock prepared acquisitions generate a new image contrast that provides molecular-level information from biological systems. Because spin-lock pulses are usually long, amplitude changes in the B1 field can occur (e.g. due to thermal effects of the RF amplifier). Such changes can create imaging artifacts that will introduce errors in the T1 quantification. Here, we present an improved approach that uses paired self-compensating spin-lock pulses to reduce artifacts from temporal inhomogeneities of the B1 field while preserving the compensation for B0 inhomogeneities.